LIGASE CHAIN REACTION WITH ENDONUCLEASE IV CORRECTION AND CONTAMINATION CONTROL

Abstract: The present invention involves methods of improving the Ligase Chain Reaction (LCR<TM>) amplification schemes by modifying at least one probe end so that the probability of the probe contributing to spurious ligation and signal development is greatly reduced. Only after specific hybridization of the modified probe with true target are the modified ends "corrected" by endonuclease IV in a target dependent fashion to allow participation of the probe in the enzymatic ligation reaction. Specific modifications include 3' phosphate blocking groups and nucleic acid overhangs containing an abasic site at the point of ligation. Further embodiments include probes modified to contain ribonucleotide moieties which, after amplification, can be cleaved by RNase to destroy the amplification products and reduce the risk of contamination.

Description: LIGASE CHAIN REACTION WITH ENDONUCLEASE IV CORRECTION AND CONTAMINATION CONTROL This application is a continuation-in-part of each of the following U.S. Applications: • Serial No.07/634,771, filed January 9, 1991, pending, which is a continuation-in- part of application Serial No. 07/470,674, filed January 31, 1990, now abandoned; • Serial No. 07/722,798, filed June 28, 1991, pending; • Serial No. 07/869,306, filed April 16, 1992, pending, which is a continuation of Serial No. 07/860,861, filed March 31, 1992, now abandoned; • Serial No. 07/925,402, filed August 3, 1992, pending; and • Serial No. 07/826,932, filed January 21, 1992, pending. The first two mentioned applications form the basis of EP-A-0439 182, published on July 31, 1991. The entire disclosure of each of the above-mentioned applications is incorporated herein by reference. BACKGROUND This invention relates to methods of amplifying target nucleic acids and, particularly, to methods of performing ligase chain reaction amplifications wherein at least one of the probes is reversibly modified at the ligation site so that it is not a substrate for the enzyme catalyzed ligation. Exemplary modifications include chemical blockage of reactant groups, or an abasic site and the addition of one or more nucleic acid bases to form an "overhang". The modified end prevents or reduces target independent spurious signal development and is later corrected in a target dependent manner to enable amplification. Oftentimes, the feasibility of a nucleic acid based diagnostic assay is dependent on the ability to amplify the signal generated by only a few molecules of target. Although signal amplification is one potential solution, target amplification is often the preferred solution in nucleic acid based assays. Target amplification involves the repeated copying or duplication of sections of the nucleic acid designated as the target sequence. One mechanism for target amplification is known as ligase chain reaction (LCR™). In LCR™, two primary probes (first and second, both of same sense) and two secondary probes (third and fourth, both of opposite sense with respect to primary probes) are employed in excess. The first probe hybridizes to a first segment of the target strand and the second probe hybridizes to a second segment of the target strand, the first and second segments being contiguous so that the 3' hydroxyl end of an "upstream" probe abuts the 5' phosphate end of a "downstream" probe, and so that a ligase can covalently ligate the two probes into a fused ligation product. In like manner, LCR™ employs upstream and downstream secondary probes. A third probe (downstream secondary) can hybridize to the first probe (upstream primary) and a fourth probe (upstream secondary) can hybridize to the second probe (downstream primary) in a similar abutting fashion. Of course, if the target is initially double stranded, the secondary probes can also hybridize to the target complement in the first instance. Once the fused strand of primary probes is separated from the target strand, it will hybridize with the third and fourth (secondary) probes which can be ligated to form a complementary, secondary fused product. In order to understand LCR™ and the improvements described herein, it is important to realize that the fused products are functionally equivalent to either the target or its complement. By repeated cycles of hybridization and ligation, amplification of the target sequence is achieved. This technique is described more completely in EP-A-320308, the entire disclosure of which is incorporated herein by reference. One of the great strengths of amplification reactions is their ability to detect exceedingly small numbers of target molecules. However, it is important that the amplification process be highly specific since the amplification of non-target sequences along with signal could potentially impair the reliability of the amplification process. One potential problem associated with ligase chain reaction is background signal caused by target independent ligation of the probes. Since the third probe hybridizes to the first probe and the fourth probe hybridizes to the second probe, the probes, which are added in excess, can easily form duplexes among themselves. These duplexes can become ligated independently of the presence of target to form a fused product which is then indistinguishable from the desired amplified target, yet which is still capable of supporting further amplification. Although target independent ligation of these duplexes is a relatively rare event, it is sufficiently common to cause undesirably high background signals in highly amplified diagnostic assays. EP-A-439 182 (corresponding to parent application Serial No. 07/634,771 ) describes several mechanisms by which this background or spurious signal in LCR™ can be reduced. One such mechanism involves 3' blocking groups or abasic sites that are "corrected" in the presence of target to yield ends that are ligation competent, i.e. ends that possess the 3' hydroxyl substrate necessary for ligation. The present invention expands and develops these mechanisms, particularly with regard to the use of endonuclease IV as the correction enzyme. Levin, et al, "Metalloenzymes in DNA Repair", /. Biol. Chem. 266(34):22893- 22898 (1991) have demonstrated that Endonuclease IV in native form contains zinc, and that inactive enzyme (purified in a metal free buffer) can be reactivated by the addition of certain divalent cations. In particular, Co 2 " 1 " and Mn 2+ at 200 μM were effective to reactivate the enzymes, depending on the method (EDTA or 1,10-phenanthroline) of inactivation. Johnson and Demple, /. Biol. Chem. 263(34): 18009- 18016 (1988) have shown that the activity of a related enzyme, yeast 3' phosphoglycoaldehyde diesterase, is enhanced by concentrations of Co 2+ from 3 μM to about 3 mM, above which the cation was inhibitory. A second potential problem associated with nucleic acid amplification systems is the potential for airborne and carryover contamination. Due to the exponential increase in target sequences, there is an increased potential for some of these molecules to contaminate an untested sample, and to render it falsely positive. Several methods have been described for reducing such contamination. They generally involve destroying substantially all the amplified products either immediately after amplification or immediately prior to the next amplification cycle. One such contamination control method is taught in co-owned, co-pending application Serial No. 07/863,622, filed April 3, 1992. Another method is taught by Walder, et al EP-A-496483. This document describes the incorporation of ribonucleotides into PCR primers followed by destruction of the amplification products with RNase or alkaline hydrolysis. While the authors allege that their method is useful in transcription based amplification and in the ligase chain reaction, they have provided no conditions or demonstration of utility except in PCR. It is well known in the art that DNA ligases will not ligate DNA probes hybridized to a ribonucleotide target. But WO91/17270 describes an LCR variation using ribonucleotide residues at the point of ligation. These residues can later be cleaved by alkali or enzymes to destroy the amplification product and prevent contamination. However, there is no teaching of using ribo-modified probes in combination with 3' blocking groups as in the present invention. The present invention provides a mechanism for reducing or eliminating contamination in LCR™ using endonuclease IV correction methods. It has been discovered that DNA probes having a single ribonucleotide bearing a 3' blocking phosphate group can be used in LCR™. When so used, the probes alleviate the background caused by target independent ligation and, at the same time, provide a mechanism to control contamination. SUMMARY OF THE INVENTION In a first aspect (the "basic modified" method), the invention relates to a method for amplifying a target nucleic acid sequence using LCR, said method including: (a) providing an excess of at least two sets of two probes, the 3' end of an upstream probe being ligated to the 5' end of a downstream probe in the presence of target to form a primary ligation product and the second set of probes hybridizing to the primary ligation product and being ligated to each other to form a secondary ligation product; (b) repeatedly denaturing the hybridized strands, reannealing additional probes and ligating them; and (c) detecting to what extent ligation products have formed, wherein the improvement comprises: (a) providing in at least one of the upstream probes a 3' end modification such that the probe is incapable of ligation to its downstream partner, said 3' end modification being correctible substantially only when the modified probe is hybridized to the target sequence; (b) hybridizing the modified probe to the target, if present, to form a modified probe-template complex; (c) correcting the modification in a target dependent manner using endonuclease IV activity to create a 3' hydroxyl end, thus allowing the corrected probe to be ligated to its downstream partner, (d) ligating the corrected probe to its downstream partner to form an amplification product; and (e) dissociating the amplification product from the target and repeating the hybridization, correction and ligating steps to amplify the desired target sequence. Preferably, the 3' modification comprises a blocking moiety (the "blocking" method) such as a blocking moiety of the form: O II — P — o-z I OH wherein Z is selected from the group consisting of -H; -(CH2)n CHO, where n is from 1 to about 3; -deoxyribose; and-dideoxyribose. A simple phosphate (Z = H) will do. The correcting solution preferably includes available divalent cobalt or manganese ion at a concentration of at least about 0.05 mM. The upper range may be lOmM or more, but more preferred ranges include from 0.05 mM to about 2.0 mM, and from about 0.5 mM to about 1.0 mM. In a variation of the blocking method,the above modified LCR method includes two sets of two probes which are oligodeoxyribonucleotide probes, except that at least one of said probes includes at least one ribonucleotide residue, preferably the terminal residue containing the 3' modification. Similar blocking modifications may be used, including phosphate at the 3' position of the terminal ribonucleotide residue. Consequently, the blocking method may further comprise, after the detection step, a step of cleaving ligation products using RNase or alkali. In addition or in the alternative, this aspect of the method may further comprise, prior to amplification, a step of cleaving ligation products using RNase. In yet another aspect (the "abasic" method), the blocking moiety may be a nucleic acid overhang containing an abasic residue immediately 3' to the point of intended liagtion. In this case, correction of the modification comprises cleavage of said modified probe on the 5' side of said abasic site, substantially only when said modified probe is hybridized to target or to ligation product. The correcting solution again preferably includes available divalent cobalt or manganese ion at a concentration of at least about 0.05 mM. The upper range may be lOmM or more, but more preferred ranges include from 0.05 mM to about 2.0 mM, and from about 0.5 mM to about 1.0 mM. In a variation of the abasic method two sets of two probes are oligodeoxyribo- nucleotide probes, except that at least one of said probes includes at least one ribonucleotide residue, preferably immediately 5' to the abasic site. Consequently, the abasic method may further comprise, after the detection step, a step of cleaving ligation products using RNase or alkali. In addition or in the alternative, this aspect of the method may further comprise, prior to amplification, a step of cleaving ligation products using RNase. In any of the above described methods, detection may be by means of a first hapten (or other specific binding member) attached to the primary upstream and secondary downstream probes; and by a reporter or second hapten (or other specific binding member) attached to the primary downstream and secondary upstream probes. In both cases, the haptens and reporters should be attached to the probes by means which do not significantly affect the hybridization and/or ligation of the probes. This may be done, for example, by attachment at the "outside ends" of the probes. Alternatively, detection may be by means of a blocking moiety which comprises a detectable label and said detecting is by means of monitoring the release of detectable label from the modified probe. Another aspect of the invention involves a diagnostic kit comprising in combination the following reagents in one or more suitable containers: (a) two pairs of probes hybridizable with target such that they are capable of being ligated in LCR, wherein at least one of the probes is modified such that, when hybridized, a ligase is substantially incapable of acting on the modified probe as its substrate; (b) a first enzyme reagent having ligase activity for assembling an amplification product; and (c) a second enzyme reagent having endonuclease IV activity capable of correcting the modified probe in a target dependent manner to allow the probe-template complex to be acted upon by the ligase reagent. The kit may also include any or all of: a) a buffer or means for preparing a buffer containing from 0.05 mM to about 2.0 mM of divalent cobalt ion; and b) an RNase reagent or an alkaline reagent. A final aspect of the invention is a nucleic acid probe substantially free of natually occurring nucleic acid fragments, said probe having at least three deoxyribonucleotides covalentiy linked by phosphodiester linkages to define a 5' and a 3' end of the probe, the probe further comprising a ribonucleotide at the 3' end, the 3' position of the ribonucleotide having attached thereto a group of the formula: O II — P — o-z I OH wherein Z is selected from the group consisting of -H; -(CH2)n CHO, where n is from 1 to about 3; -deoxyribose; and-dideoxyribose. Preferably, Z is hydrogen and the oligonucleotide comprises from about 12 to about 50 deoxyribonucleotides. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a graphic representation of the prior art blunt-ended LCR™. Primary probes A and B hybridize to target and become ligated. Secondary probes A' and B' hybridize to target complement or to fused primary probe product and similarly become ligated. Figure 2 is graphic representation of the improved LCR™ according to the invention. Upstream probe A is modified to include blocking phosphate groups on its 3' end where a 3' hydroxyl is needed for ligation. Correction (phosphate removal by Endonuclease IV when probe A is hybridized to target) leaves a 3' hydroxyl and ligation to downstream probe B is possible Figure 3 is another graphic representation of the improved LCR™ according to the invention. In this case, abasic site modified upstream probe A cannot be ligated directly to its downstream partner B; but after correction (cleavage at the abasic site "x" by Endonuclease IV when probe A is hybridized to target) a 3' hydroxyl is restored and ligation to B is possible. Figures 4-6 show gels depicting data from examples 5 and 13-15. They are described in greater detail in the examples. DETAILED DESCRIPTION For purposes of this invention, the target sequence is described to be single stranded. However, this should be understood to include the case where the target is actually double stranded but is simply separated from its complement prior to hybridization with the probes/primers. In the case of double stranded target, secondary, third and fourth probes will also participate in the initial step by hybridizing to the target complement. In the case of single stranded target, the secondary probes will not participate in the initial hybridization step, but will participate in subsequent hybridization steps. Target sequences may comprise deoxyribonucleic acid (DNA) or ribonucleic acid (RNA). It is to be understood that the term "bases" shall refer to Guanine (G), Cytosine (C), Adenine (A) and Thymine (T) when the context is that of DNA; and Guanine (G), Cytosine (C), Adenine (A) and Uracil (U) in the context of RNA. The term also includes analogs and derivatives of the bases named above, provided they can undergo hydrogen bonding of base pairs characteristic of the natural bases. Exemplary base "analogs" can be found in 1114 Official Gazette, at 43. Although the degenerate base Inosine (I) may be employed with probes used in this invention, it is not preferred to use I within modified portions of the probes according to the invention. Individual nucleotides or bases are said to be "complementary" if they result in canonical base pairing; e.g. C with G, and A with T or U. Throughout this application, the "prime" (') designation is used to indicate a complementary base or sequence. One oligonucleotide is "complementary" to another if it hybridizes to the other and has substantially complementary base pairings in the hybridized region. Thus, probe A can be complementary to A' even though it may have ends not coterminal with A'. The same is true of B and B'. As a result of this definition, "complementary" oligonucleotide sequences encompass sequences that have mismatched base pairs in the hybridizable region, provided they can be made to hybridize under assay conditions. It is an important feature of the present invention that, instead of using complementary pairs of probes capable of forming ligatable, blunt-ended duplexes, at least one probe of one of the probe pairs initially includes a "modified" end which renders the resultant primary or secondary probe not a suitable substrate for the ligase catalyzed fusion. A "modified end" is defined with respect to the point of ligation rather than with respect to its complementary probe. A "modified end" has either (1) a blocking moiety (or additional base residues) on a group (e.g. the 5' phosphate or the 3' hydroxyl) which, under ordinary LCR™ conditions, obligatorily participates in the ligase catalyzed fusion (See e.g. probe A of Figure 2); or (2) omitted bases to create a "gap" between one probe terminus and the next probe terminus. By convention in this application, a modified end of the first type is referred to as an "overhang", the overhang being an additional blocking moiety or additional base residues which, when hybridized to the target sequence extends beyond the point of ligation. The term "overhang" is not to be confused with an "extension" of one probe with respect to its complementary probe, resulting from the fact that they need not be coterminal. A modified end of the second type is referred to herein as a "recess", the recess being the gap between two primary or secondary probes after hybridizing to the target The presence of these modified ends reduces the falsely positive signal created by target independent ligation of complementary probe duplexes to one another in the absence of target. "Correction" of the modification is subsequently carried out to render the probes ligatable. As used herein "correction" refers to the process of rendering, in a target dependent manner, the two primary probes or the two secondary probes, or both, ligatable to their same sense partners. Thus, only those probes hybridized to target, target complement or polynucleotide sequences generated therefrom are capable of being "corrected". "Correction" can be accomplished by several procedures, depending on the type of modified end used, although endonuclease IV corrections are examined herein. As used herein, "point of ligation" or "intended point of ligation" refers to a specific location between two probe partners that are to be ligated in a template-dependent manner. It is the site at which the "corrected" upstream probe lies adjacent its downstream partner in 5' phosphate- 3' hydroxyl relationship. For each set of four LCR™ probes there are two "points of ligation", a point for the primary probe partners and a point for the secondary probe partners. In conventional LCR™, typically the two points of ligation are opposite one another, thus forming blunt ended duplexes when the probe pairs hybridize to one another. In the present invention, the points of ligation may be opposite one another or displaced from one another (preferably with 3' extensions) by one or more bases. The exact point(s) of ligation varies depending on the embodiment and, thus, this term is further defined in the context of each embodiment. Each of the probes may comprise deoxyribonucleic acid (DNA) or ribonucleic acid (RNA). It is a routine matter to synthesize the desired probes using conventional nucleotide phosphoramidite chemistry and the instruments available from Applied Biosystems, Inc, (Foster City, CA); DuPont, (Wilmington, DE); or Milligen, (Bedford, MA). Phosphorylation of the 5' ends of the probes, a necessity for ligation by ligase, may be accomplished enzymatically by a kinase, as is known in the art, or by any chemical synthetic method known to phosphorylate 5' ends. Commercial reagents are available for this purpose for use with automated synthesis. As will be seen, similar methods and reagents are used to place a blocking phosphate on a 3' probe end. In general, the methods of the invention comprise repeated steps of (a) hybridizing the modified probes to the target (and, if double stranded so that target complement is present, to the target complement); (b) correcting the modification in a target dependent manner to render the probes ligatable; (c) ligating the corrected probe to its partner to form a fused or ligated product; and (d) dissociating the fused product from the target and repeating the hybridization, correction and ligation steps to amplify the desired target sequence. Steps (a), (c) and (d) are essentially the same for all of the embodiments and can be discussed together. They are generally the same steps that one would employ in conventional LCR™. Step (b) varies depending on the type of modification employed and correction by endonuclease IV is discussed herein. Hybridization of modified probes to target (and optionally to target complement) is adequately explained in the prior art; e.g EP-320308 and EP 439 182. Probe length, probe concentration, GC content and stringency of conditions all affect the degree and rate at which hybridization will occur. Preferably, the probes are sufficiently long to provide the desired specificity; i.e, to avoid being hybridizable to random sequences in the sample. Typically, probes on the order of 15 to 100 bases serve this purpose. Presently preferred are probes having a length of from about 15 to about 40 bases. The probes are generally added in approximately equimolar concentration since they are expected to react stoichiometrically. Each probe is present in a concentration ranging from about 5 nanomolar (nM) to about 90 nM; preferably from about 10 nM to about 85 nM. The optimum quantity of probe used for each reaction also varies depending on the number of cycles which must be performed. Optimum concentrations can readily be determined by one of ordinary skill in this art The stringency of conditions is generally known to those in the art to be dependent on temperature, solvent and other parameters. Perhaps the most easily controlled of these parameters is temperature and thus it is generally the stringency parameter varied in the performance of LCR™. Since the stringency conditions required for practicing this invention are not unlike those of ordinary LCR™, further detail is deemed unnecessary, the routine practitioner being guided by the examples which follow. The next step in the general method follows the specific correction step and comprises the ligation of one probe to its adjacent partner. Thus, primary upstream probes are ligated to their associated primary downstream probes and secondary upstream probes are ligated to their associated secondary downstream probes. An "adjacent" probe is either one of two probes hybridizable with the target in a contiguous orientation, one of which lies with its phosphorylated 5' end in abutment with the 3' hydroxyl end of the partner probe. "Adjacent" probes are created upon correction of the modified end(s) in a target dependent manner. Since enzymatic ligation is the preferred method of covalentiy attaching two adjacent probes, the term "ligation" will be used throughout the application. However, "ligation" is a general term and is to be understood to include any method of covalentiy attaching two probes. The conditions and reagents which make possible the preferred enzymatic ligation step are generally known to those of ordinary skill in the art and are disclosed in the references mentioned in background. Ligating reagents useful in the present invention include prokaryotic ligases such as E coli ligase, T4 ligase and Thermus thermophilus ligase (e.g., ATCC 27634) as taught in ΕP-320 308 and EP-A-373 962. This latter ligase is presently preferred for its ability to maintain activity during the thermal cycling of LCR™. Suitable thermally stable ligases are commercially available from New England Biolabs, Inc. (Beverly, MA), Epicentre Technologies, Inc.(Madison, WI) and Molecular Biology Resources (Milwaukee, WI). Absent a thermally stable ligase, the ligase must be added again each time the cycle is repeated. Also of potential utility are eukaryotic ligases, e.g. DNA ligase of Drosophilia, reported by Rabin, et al, /. Biol. Chem. 261:10637- 10647 (1986). Once ligated, the fused probe is dissociated (e.g. melted) from the target and, as with conventional LCR™, the process is repeated for several cycles. The number of repeat cycles may vary from 1 to about 100, although from about 15 to about 70 are preferred presently. It is desirable to design probes so that when hybridized to their complementary (secondary) probes, the ends away from the point of intended ligation (i.e. "outside" ends) are not free themselves to participate in other unwanted ligation reactions. Thus, ligatable sticky or blunt outside ends should be avoided. Free 5' terminal phosphates should be avoided or eliminated, especially if such sticky or blunt ends must be used. This can be accomplished either through synthesizing oligonucleotide probes (which normally carry no 5' terminal phosphate groups), or through the use of phosphatase enzymes to remove terminal phosphates (e.g. from oligonucleotides generated through restriction digests of DNA). Alternatively, ligation of the outside ends of the probes can be prevented by blocking the end of at least one of the probes with a "hook" or other reporter molecule or marker moiety as will be described in detail below. It is also desirable to design the probes so that substantially all of the amplification products made can be selectively inactivated or destroyed to reduce the risk of contamination. To be effective, such inactivation must destroy substantially all the amplification products and it may occur at various times during the process. Generally inactivation may occur immediately after amplification, after detection or immediately prior to the next LCR™ reaction. For convenience these will be referred to herein as "post- amplification", "post-detection" and "pre-amplification", respectively. An inactivation method that is used pre-amplification must selectively destroy amplification product without destroying the reactant probes and reagents. An inactivation method that is used post-detection does not share this constraint. For example, a ribonucleotide residue can be incorporated into an otherwise deoxyribo-oligonucleotide and the resultant product can be cleaved by RNase or alkaline hydrolysis conditions. Particularly useful among the RNases reported in the literature are RNaseH, which cleave ribonucleotides in RNA:DNA duplexes and in single stranded RNA. Although E coli. Rnase generally prefers a string of about 4 ribonucleotides for efficient cutting, Walder, et al. have reported a human RNase H activity in K562 erythroleukemia cells, designated RNaseHI (see EP-A-496483), which is said to cleave mixed R/DNA:DNA duplexes when only a single ribo residue is present. The procedure by which this is accomplished is described in greater detail in EP-A-496483 and in WO91/17270. It is also possible to cleave the ribo-modified products using alkaline conditions. In the context of this application, "alkaline conditions" refers to conditions above a pH of 7.0 which are sufficient to effect a hydrolysis of the phosphodiester bond adjacent a ribonucleotide moiety. Usually a pH above about 10 for 0.5 to 2 hours with heat will produce hydrolysis. Typical conditions are known in the art and include by way of example, not limitation, treatment with 0.6N NaOH for 30 min to 1 h at 90° C or with 30- 40 mM KOH or NaOH for 1-1.5 hours. A 36 mM solution of KOH produces a pH of about 11. Other probe modifications that permit destruction after ligation are also within this invention. MODIFIED ENDS CORRECTIBLE BY ENDONUCLEASE IV As mentioned, a first embodiment involves a modified end wherein a blocking moiety or additional bases are added to the 3' end of at least one upstream probe, beyond the point of intended ligation. The blocking moiety or the additional bases comprise the "overhang" and are the reason blunt-end ligation is not possible. Modified Probe Reagents. In a first variation depicted schematically in Figure 2 and exemplified in Examples 5-7, the overhang comprises a chemical blocking agent, R. It is well known that the standard DNA ligase reaction requires that the substrate strands present a 3' hydroxyl and a 5' phosphate at the point of ligation. Several modifications, particularly at the 3' hydroxyl group, are known to introduce an R group which will render the modified end incapable of participating in a ligase reaction, but which can be removed when the modified strand is part of a double stranded structure. Such modifications include the following illustrative R groups attached to the 3' oxygen in place of the hydrogen atom: O II — P — o-z I OH wherein Z is selected from the group consisting of -H;-(CH2)n CHO, where n is from 1 to about 3, preferably 1 or 2;.-deoxyribose; and -dideoxyribose. The synthesis of probes having ends suitably modified with an R group is well known in the art. For example, chemical synthesis of oligonucleotides containing a 3' phosphate group has been described by Markiewicz and Wyrzykiewicz, Nucl. Acids Res. 17:7149-7158 (1989). Larger blocking groups, which may have the advantage of hiding the 3' phosphate from non-specific phosphatases that might be present in some samples, are conveniently prepared by creating the oligonucleotide probe with terminal transferase and dUTP or ddUTP, followed by treatment with uracil glycosylase. Purification of uracil glycosylase is taught by Lindahl, et al, J.B.C. 252: 3286-3924 (1977). In the case of dUTP addition, treatment with strong alkali following the uracil glycosylase treatment can be used to prepare the glycoaldehyde derivative. It is to be understood that the examples of R groups given above are illustrative only, and that one of ordinary skill could synthesize many variants which would work equally well. In addition, example 16 describes a convenient automated synthesis of a probe having a 3' terminal phosphate. In another variation, the probes are modified to contain both a 3' ribonucleotide moiety and a 3' phosphate blocking moiety. Synthesis of such probes is described in Examples 14-17. These probes have dual advantages. They avoid the problem of target independent ligation by virtue of the 3' blocking phosphate. In addition, they avoid the problems of contamination because the amplification products can be cleaved and destroyed at the ribonucleotide residue by various RNases or alkaline conditions. Any contamination control method that cleaves the amplification product may be used post-detection. Either RNase or alkaline conditions can be used to cleave the hgation product at the ribonucleotide when the reaction is completed. However, for pre- amplification contamination control, RNase is preferred. While alkaline conditions will cleave ligation products to leave a 5' hydroxyl, it also demonstrates a tendency to act on unligated ribo-modified probe reagents to convert 3' phosphates to 2' phosphates. It is not known whether or how efficiently the 2' phosphated probes can be ligated. In contrast, preferred RNases cut the ligation products leaving a 5' hydroxyl and a 3' phosphate. In other words, RNase cleaves a phosphodiester bond on the 3' or downstream side of the ribo residue, a bond that does not even exist in the unligated modified reagents. Thus, RNase can be used as a pre-amplification contamination control without damaging the amplification reagents which still need to perform. The RNase may be, and preferrably is, thermolabile since it should not retain activity after LCR™ begins. In yet another variation of overhanging ends, the overhang consists of additional nucleic acid bases which can be cleaved off once the probes are hybridized to target. This variation is depicted schematically in Figure 3 and exemplified in Examples 3-4 and 8-9. The overhang prevents ligation at the intended point of ligation by virtue of its bulk, and stereochemically blocks or masks the group(s) which obligatorily participate in the ligase reaction (as described above for blocked ends). What distinguishes this from the simple chemical blockage described above is the nature and size of the "blocking" group (i.e., the overhang). It is by nature composed of nucleic acid residues colinear with the probe molecule. However, the size of the group is too large to permit the modified end of the molecule to remain in the vicinity of the ligation point when hybridized to a target. Although several classes of overhangs are possible, (three are described in the parent applications) the phosphate-blocked and abasic site overhangs are further considered herein. In general, the overhang should be complementary to the target so that its removal can be template dependent as is described below. The overhang may be from 1-10 bases, preferably from 1-5 bases in length. The synthesis of oligonucleotides with abasic sites has been described in the literature. See, for example, Takeshita, et al, J iol.Chem. 262:10171-10179 (1987); and Eritja, et al. Nucleosides & Nucleotides 6(4):803-814 (1987). Modified oligonucleotide probes can be synthesized so as to position an abasic site immediately 3' to the point of ligation on the probe intended to donate its 3' end. For both blocked and abasic modified probes, the probes are preferably designed to minimize correction that might take place while the probe is hybridized to its complementary probe (as opposed to correction while hybridized to true target). With reference to Figure 2 for example, when the 5' terminus of A' is coterminal with the 3' phosphate blocked probe A there may be some tendency for endonuclease IV to recognize this as its double stranded substrate and to cleave a 3' phosphate from A even in the absence of target. Similarly, in Figure 3, when the 5' terminal base of A' is opposite the abasic site, x, of probe A, and even when opposite the residue immediately 5' of the abasic residue in probe A, there may be some tendency for endonuclease IV to recognize this as its double stranded substrate and to cleave the overhang from A at the abasic site even in the absence of target. Both of these adverse situations can be minimized by staggering or offsetting the ends of modified probes A (and/or B') so that they extend as single strands beyond the ends of their complementary probes. Furthermore, in the case of the abasic modification, the abasic site itself preferably lies beyond the 5' end of the complementary probe by at least one or more bases. In other words, the 5' terminal residue of the complementary probe A' lies opposite a nucleotide residue in the upstream probe A which is at least one residue 5' to the site of the modification. In the phosphate blocked situation, this is illustrated by example 7, using probe AA123- 1P(20) complementary to AA123-3(18), and probe AA 123-4P(22) complementary to AA 123-2 (a 20-mer). Similarly, in the abasic site situation, this is demonstrated by example 8 using probes AA123-1E1 complementary to AA123-3(18), and probe AA123-4E1 complementary to AA123-2 (a 20-mer). In each of these cases, the 3' modification extends beyond the 5' end of the complementary probe, virtually assuring that endonuclease IV will not mistake this duplex for its true substrate, modified probe on complete target strands. Enzymology. The enzyme endonuclease IV (Siwek, et al, Nucl. Acids Res. 16:5031-5038 (1988) sometimes referred to herein as "Endo IV") and a variety of other naturally occurring enzymes are capable of removing various blocking groups to expose a 3' hydroxyl group if and substantially only if the strand containing the blocking group is hybridized to a complementary strand. For example, Doetsch and Cunningham, Mutation Research, 236:173-201 (1990) describe in detail the enzymology of endonucleases and the chemistry of several different reactive abasic sites. The same enzyme has also been shown to cleave a polynucleotide at an abasic site if the polynucleotide is hybridized to a complementary strand. Endonuclease IV is a class II AP endonuclease, which effects cutting on the 5' side of the abasic site, leaving a 3' hydroxyl end on the polynucleotide. By both its position and its chemical nature, the polynucleotide is now capable of being joined by ligase to an adjacent probe. While use of the endonuclease IV referred to herein is clearly within the scope of the invention, it will be recognized that many other equivalent correcting reagents may be employed. For example, other enzymes may be discovered that have a similar ability to correct the modifications substantially only when the modified probe is hybridized to target Also, it may be found that the entire enzyme is not essential, but that some fragment or digest of the enzyme will have the desired activity. Finally, it may be that desired activity may be obtained by recombinantly produced polypeptides having only a fraction of the length of the native protein. All such variations are deemed equivalents of Endonuclease IV for purposes of this invention. If the endonuclease IV enzyme is not thermostable, it should be re-added at each cycle of LCR™. However, it is preferred to use endonuclease isolated or recombinantly engineered from a thermostable species, such as the T. thermophilus Endo IV described in copending and co-owned U.S. applications Serial Nos. 07/860,861 and 07/869,306, filed on March 31, 1992 and April 16, 1992, respectively, the entire disclosures of which have been incorporated herein by reference. E. coli strain CS1 carries plasmid pTT7 containing an insert coding for this enzyme was deposited with the ATCC on April 9, 1992 and has been given Accession No.68950. In the context of the present application, "thermostable" means that the enzyme reagent retains a substantial portion of its activity at temperatures in excess of about 70° C; preferably in excess of about 80° C. It has been shown by Levin, et al, "Metalloenzymes in DNA Repair", /. Biol. Chem. 266(34) :22893-22898 (1991) that Endonuclease IV in native form contains Zinc. These researchers have also shown that endonuclease IV which has been purified in a metal free buffer is inactive, but it can be reactivated by the addition of certain divalent cations. In particular, Co 2-1 " and Mn 2+ at 200 μM were effective to reactivate the enzymes, depending on the method (EDTA or 1,10-phenanthroline) of inactivation. Others have shown that the activity of a related enzyme, yeast 3' phosphoglycoaldehyde diesterase, is enhanced by concentrations of Co 2+ from 3 μM to about 3 mM, above which the cation was inhibitory. However, it has suprisingly been found that Co 2+ is inhibitory to the performance of ligase and LCR™ generally. For example, see Examples 12 and 13, infra, wherein concentrations above about 2 mM prevented all amplification from IO 6 targets. Thus, it was necessary to find the window of concentration of Co 2 " 1 " which enabled the endonuclease IV without inactivating the LCR™ (apparently the ligase itself). Since Co 2+ is very tightly bound by chelators such as EDTA, and it is difficult to completely remove EDTA often used in the preparation of samples, it is useful to refer to "available" cobalt ion, which is the amount in excess of that bound by any chelator present. If careful measures are taken to avoid introducing or to completely remove chelator, then the "available" cobalt ion concentration approximates the actual concentration. It is believed that the available concentration of Co 2 * should be about 0.05 to about 2.0 mM, usually about 0.1 to about 1.5 mM, preferably about 0.5 to about 1 mM. Cobalt ion is suitably supplied as any common salt, such as the dichloride salt. Applicants have also discovered surprisingly that the divalent cation Mn 2+ will substitute for Co 2 " 1 " but not for Mg 2+ in the LCR™ involving endonuclease IV. This is surprising because literature reports establish that Mn 2+ , as the sole divalent cation, will support ligase activity and endonuclease IV activity. However, Mn 2+ alone is not sufficient to support the LCR or endo IV modified LCR. The presence of Mg 2+ is still a requirement for the amplification reactions. Preferably the Mg 2+ concentration is at least about 0.5 mM, and ideally is from about 5 to about 20 mM. Detection. Following amplification, the amplified sequences can be detected by monitoring the formation of ligated product using a number of conventional technologies known in the art. In one prefeired way, formation of the ligated product is monitored using the fact that a new covalent bond is formed between the first and second probes of the same sense. Thus, amplification product is longer than the individual probes and can be separated form unligated probes on this basis. Separation may easily be achieved by gels or by affinity members or "hooks". A "hook" is any moiety having a specific ligand-receptor affinity. It may be, for example, a hapten or a segment of polynucleotide. A hook may be attached to one probe and a label may be attached to the other probe of same sense. Ligation joins the label to the affinity moiety and separated label can be measured on a solid phase following separation. Alternatively, hooks may be provided at the available outside ends of at least two probes (opposite ends of fused product), and preferably to the outside ends of all four probes. Typically, the hook(s) at one end of the fused product (e.g. the 5' end of A and the 3' end of A') comprises a first hapten capable of being immobilized by a reagent (such as antibody or avidin) coated onto a solid phase. The hook(s) at the other end (e.g. the 3' end of B and the 5' end of B') contains a different antigen or hapten capable of being recognized by a label or a label system such as an antibody-enzyme conjugate. In the case of an enzyme conjugate, a substrate is then added which is converted by the enzyme to a detectable product. Exemplary hapten hooks include many drugs (eg. digoxin, theophylline, phencyclidine (PCP), salicylate, etc.), T3, biotin, fluorescein (FITC), dansyl, 2,4- dinitrophenol (DNP); and modified nucleotides such as bromouracil and bases modified by incorporation of a N-acetyl-7-iodo-2-fluorenylamino (AIF) group; as well as many others. Certain haptens are disclosed in co-pending, co-owned patent applications U.S. 07/808,508 (adamantaneacetic acids), U.S. 07/808,839 (carbazoles and dibenzofurans), both filed December 17, 1991, U.S. 07/858,929 (acridines), and U.S. 07/ 858,820 (quinolines), both filed March 27, 1992 (collectively referred to herein as the "hapten applications"). The entire disclosure of each of the above hapten applications is incorporated herein by reference. Virtually any hapten can be used with the present invention. The invention requires only that a specific binding partner is known or can be prepared (a definitional property of "hapten") and that the hapten can be coupled to the probe such that it does not interfere with hybridization or ligation. Many methods of adding haptens to probes are known in the literature. Enzo Biochemical (New York) and Clontech (Palo Alto) both have described and commercialized probe labelling techniques. For example, a primary amine can be attached to a 3' oligo end using 3'-Amine-ON CPG™ (Clontech, Palo Alto, CA). Similarly, a primary amine can be attached to a 5' oligo end using Aminomodifier π® (Clontech). The amines can be reacted to various haptens using conventional activation and linking chemistries. Publications WO92 10505, published 25 June 1992 and WO 92/11388 published 9 July 1992 teach methods for labelling probes at their 5' and 3' ends respectively. According to one known method for labeling an oligonucleotide, a label-phosphoramidite reagent is prepared and used to add the label to the oligonucleotide during its synthesis. For example, see Thuong, N. T. et al., Tet. .Letters, 29(46):5905-5908 (1988); or Cohen, J.S. et al., U.S. Patent Application 07/246,688 (NTIS ORDER No. PAT-APPL- 7-246,688) (1989). In another embodiment of this invention, detection is achieved not by directly measuring formation of the ligated product, but by measuring release of the blocking moiety or the overhang. This can easily be done if the blocking moiety or overhang contains a detectable label. In this case, a reduction in signal associated with the solid phase indicates the presence of target. In a preferred variation the label is a fluorophore having a first characteristic spin property when attached to the probe, and a second, distinguishable spin property when released from the probe. Such labels are well known in the art of fluorescence polarization assays. See, for example, EP-A-382433 (ICI). Coupling these labels to blocking moiety or to a nucleic acid overhang is a matter of routine chemistry. EXAMPLES The invention will now be described further by way of examples. The examples are illustrative of the invention and are not intended to limit it in any way. Examples 1- 2 below, illustrate enhanced LCR™ using probes having modified ends which are corrected by endonuclease IV. Both blocking phosphate groups and abasic site overhangs are demonstrated. The probe sequences are given in Tables I - IV below. The probes are specific for a target DNA sequence at position 123 or 250 in Actinobacillus actinomycetemcomitans, hence the designation "AA123" or "AA250". The next numeral (after the dash) designates the position of the probe in a set of four: Probes numbered -1 and -2 have the same 5'-3' orientation while probes numbered -3 and -4 have the opposite sense. Probes numbered -1 and -3 hybridize, as do probes numbered -2 and -4. "P" and "p" indicate a phosphate group. This is normal and required on a 5' terminus, but serves as a ligation blocking modification on a 3' end. 3' phosphorylated probes were synthesized by initiating automated synthesis with 2-[[2-[(4,4'- dimethoxytrityl)oxy]ethyl] sulfonyl]ethyl 2-cyano-ethyl N,N-diisopropylphosphor- amidite (Horn, T and Uidea, M Tet. Lett. 27 4705+ (1986)) as reported (Ashely, GW and Kushland, DM Biochemistry 30: 2927-2933 (1991)), followed by the sequential addition of ribonucleotide and deoxyribonucleotide cyanoethyl phosphoramidites using an automated DNA synthesizer. "E" and "x" designate an abasic site (described further below). A number (1, 3 or 5) following the "E" designates the length of complementary bases (overhang) beyond the abasic site. Numbers in parentheses represent probe lengths. Abasic probes were synthesized on automated instruments using modified phosphoramidite reagents according to the method oiΕήt _ι, c_t _ . Nucleosides & Nucleotides 6{ ) %Q -%\A (1987). All probes are oligodeoxyribonucleotides except as specified. The target DNA used in examples 1-12 and 14-17 was a plasmid containing an 898 base pair insert isolated from Actinobacillus actin mycetemcomitans. (ATCC Ace. No. 53219). The plasmid was digested with EcoRI and PstI to liberate an approximately 1000 base pair fragment. Plasmid concentrations were determined spectrophotometrically assuming an O.D. equal to 1.0 corresponds to a DNA concentration equal to 50 μg/mL. Target DNA solutions were made by serially diluting the digested plasmid in 5 mM Tris pH 7.8, 0.1 mM EDTA and 300 μg/mL human placental DNA. All reactions, unless otherwise stated, were performed in LCR Buffer (45 mM EPPS pH 7.8, 80 mM KC1, 10 mM MgCl 2 , 10 mM NH4CI and 0.5 mM NAD + ) supplemented with acetylated bovine serum albumin (BSA) and temperature cycling was achieved with a COY Model 50 temperature cycler. Reactions were terminated by transferring aliquots into Stop Buffer (80% formamide, 20 mM EDTA, 0.05% (w:v) xylene cyanol and 0.05% bromophenol blue). The ligated and unligated products were resolved on a 16x20x0.04 cm 15% polyacrylamide gel containing 8.3 M urea in 80 mM Tris, 80 mM boric acid pH 8.0, 1.0 mM EDTA. The gel was autoradiographed, the autoradiograph used as a template to excise the ligated and unligated probes and the amount of radioactivity in each band was measured by liquid scintillation counting. The percentage of radioactivity in the ligated product was calculated as a function of the total counts in each lane. Unless otherwise stated, the following abbreviations have the meaning indicated. BSA bovine serum albumin EDTA a metal chelator, ethylenediamine tetraacetic acid EPPS a buffer comprising N-(2-hydroxyethyl)piperazine-N'-(3- propanesulfonic acid) HPLC high performance liquid chromatography NAD or NAD+ nicotine adenine dinucleotide Tris a buffer comprising tra(hydroxymethyl)aminomethane TTh Thermus thermophilus Table I. AA PROBE SETS TO Position 123 of ACTINOBACILLUS ACTINOMYCETEMCOMITANS Designation SEOUENCE SEO ID No. AA123-K20 ) -TTGTCGAGCACCTTGAATAA 1 AA123-1P (20 ) -TTGTCGAGCACCTTGAATAAp 2 r -^ | AA123-1-E1 -TTGTCGAGCACCTTGAATAAxT 3 AA123-1-E3 -TTGTCGAGCACCTTGAATAAxTAA 4 AA123-1-E5 -TTGTCGAGCACCTTGAATAAxTAATG 5 5 ' 3 ' AA123-2 pTTAATGGCTTCGATTGGGCT- 6 3 ' -5 1 AA123-3 (20 ) -AACAGCTCGTGGAACTTATTp 7 AA123-3 (18 ) -AACAGCTCGTGGAACTTAp 8 3 ' 5 ' AA123-4P (22 ) pTTAATTACCGAAGCTAACCCGA- 9 AA123-4P (20 ) pAATTACCGAAGCTAACCCGA- 10 AA123-4 (20 ) AATTACCGAAGCTAACCCGA- 11 AA123-4-E1 TxTTAATTACCGAAGCTAACCCGA- 12 AA123-4-E3 CTTxTTAATTACCGAAGCTAACCCGA- 13 AA123-4-E5 AACTTxTTAATTACCGAAGCTAACCCGA- 14 where x = abasic site; p = 3' - phosphate blocking group; and p = 5' - phosphate group (normal ligation substrate) Table II. AA PROBE SETS TO Position 250 of ACTINOBACILLUS ACTINOMYCETEMCOMITANS Designatipn SEQUENCE SEOIDNo. 5' AA250-1 -CCGATTGCAATGTAATATCGACGTC 15 5« AA250-1E5 -CCGATTGCAATGTAATATCGACGTCxTCGGC 16 AA250-2 pGTCGGGCAAATAATTCGCCAC- 17 3' 5' AA250-3(24) - GCTAACGTTACATTATAGCTGCAGp - 18 3' 5' AA250-3(22) - GCTAACGTTACATTATAGCTGCp - 19 3' 5' AA250-4(19) - CAGCCCGTTTATTAAGCGG - 20 3' 5' AA250-4(21) - AGCAGCCCGTTTATTAAGCGG - 21 where x = abasic site; p = 3' - phosphate blocking group; and p = 5' - phosphate group.(normal ligation substrate) Table III. Synthetic Target Molecules For AA123 Probe Sets 5' 3 1 AA123TAR-3/4 -TTGTCGAGCACCTTGAATAATTAATGGCTTCGATTGGGCT- 22 3' 5' AA123TAR-1/2 -AACAGCTCGTGGAACTTATTAATTACCGAAGCTAACCCGA- 23 Substrate For Abasic Nuclease Detection E4SUB1 A C^CGGATCCGGCTxTTTTTGGGG-S ' 24 C AτG CCTAGGCCGAAAAAAA-5' where x = abasic site. EXAMPLE 1: Blunt-End LCR™ LCR™ was performed using the blunt-ended probe set (see Table I) consisting of AA123-1(20), AA123-2, AA123-3(20) and AA123-4(20) in a 20 μL reaction volume containing LCR Buffer supplemented with 10 μg/mL BSA and 300 ng of human placental DNA. Each probe was present at 83 nM (approximately 5% of probe 2 was 3 '-end labeled with [α- 32 P]-cordycepin triphosphate to enable detection) and the final concentration of Thermus thermophilus (Tth) DNA ligase equalled 0.15 μg/mL. Duplicate reactions containing either zero or IO 6 molecules of target DNA were performed. The samples were overlaid with 15 μL of mineral oil and the temperature cycle consisted of a 90 β C incubation for 30 seconds followed by a 50 β C incubation for 30 seconds. At specified cycles (see Table E-l), 1.4 μL aliquots were removed, mixed with 2.0 μL of stop buffer, heated to 90°C for 2 min and applied to a denaturing polyacrylamide gel. Table E- 1 shows the average percent ligated and the ratio of (+)/(-) target for the duplicate reactions. The data show that IO 6 targets are distinguishable from zero targets. However, other data show that detection of fewer than 10 5 were not adequately reproducible using unmodified probes. Table E-l: Blunt-End LCR Percent Ligated Cvcle No. 0 Targets 106 Targets Ratio (+/-) Target 25 1.65 32.17 19.5 28 5.76 41.88 7.3 31 14.55 52.27 3.6 34 23.28 59.97 2.6 37 34.11 65.20 1.9 EXAMPLE 2: Demonstration of Blocking Effect of Abasic Modification on Ligation 3'[α- 2 P]-cordycepin labeled AA123-2 was incubated at 50 β C for 40 min with either AA123-1E1, AA123-1E3, or AA123-1E5 (see Table I), the complementary sequence AA123TAR(l/2)(Table HI), and Tth DNA ligase in the presence (+) or absence (-) of Tth endonuclease IV. An identical set of reactions employing the AA123-4E1, AA123-4E3 and AA123-4E5 (Table I) with AA123TAR(3/4) (Table HI) and 3'[α- 2 P]- cordycepin labeled AA123-3(18) was also performed. The ligated and unligated products were resolved on a 20x40x0.04 cm 12.5% polyacrylamide gel containing 8.3 M urea in 80 mM Tris, 80 mM boric acid pH 8.0, 1.0 mM EDTA. Figure 4 is an autoradiograph showing that the abasic probes with abasic extensions are not suitable substrates for DNA ligase; and that the blocking extensions are removed by endonuclease IV to render probes -1 and -4 ligatable to probes -2 and -3, respectively. Other data (not shown) confirm the same results are achieved with 3'-PO4 blocking groups and furthermore, the correction and/or ligation of the probes with endonuclease IV and/or DNA ligase occurs only in the presence of the correct complementary sequence. Examples 3-4, and 8-9 relate to LCR reactions using probes modified to contain abasic sites followed by extensions. Examples 5-7 relate to LCR reactions using probes modified to contain 3' blocking phosphate groups. EXAMPLE 3: LCR Using One Abasic Site Probe LCR was performed using probes AA123-1(20), AA123-2 and AA123-3(18) and AA123-4E5 from Table I in a Coy model 50 thermocycler set for 95°C for 30 sec, followed by 55 β C for 110 sec. Reactions were run in 20 μL volume containing 45 mM EPPS pH 7.8, 80 mM KCl, 10 mM MgCl 2 , 0.5 mM NAD+, 1 mM CoCl 2 , and 300 ng human placental DNA. Unless otherwise indicated, probes were present at 83 nM (about 5% of probe AA123-2 was 3' end labeled with radioactive [α- 32 P]-cordycepin triphosphate to enable detection) and Tth ligase (varying amounts, see data table E-3). Target was zero, IO 2 , 10 3 or IO 4 molecules of EcoRl and PstI digested AA DNA as shown. The data is presented in Table E-3 below as a signal-to-background ratio. A signal which is consistently at least 3-4 times that of background is generally sufficient to distinguish target from background. Table E-3: Signal- to-Background Ratio Endo IV Fourth Probe Dilution Cycle No. 1 0 3 Targets 1 (^ Targets 1 0 5 Targets AA123-4E5 1:5000 30 12.0 AA123-4E5 1:2000 30 9.7l AA123-4E5 1:10000 30 22.4 2 AA123-4E5 1:50000 30 1 3.5 11.6 40 3.1 6.5 9.8 l - Under identical conditions, unmodified probe AA123-4 produced a S/B ratio of only 1.6. 2 - Under identical conditions, probe AA123-4E3 also produced a S B ratio of 20 at 30 cycles. EXAMPLE 4: Comparison of Different Abasic Site Probes Example 3 is repeated, except probe concentration was reduced to 17 nM in a reaction volume of 50 μL, and the fourth probe is either AA123-4E5 or AA123-4E3. The data is shown below, as a signal-to-background ratio. Table E-4: Signal-to-Background Ratio Endo IV Fourth Probe Dilution Cycle No. IO 3 Targets IO 5 Targets AA123-4E3 1:5000 40 1 L 6.7 AA123-4E5 40 1 L 5.0 AA123-4E3 1:10000 40 1 1 7.7 45 1 i 6.2 AA123-4E5 40 1 I 11.5 45 1 i 5.0 EXAMPLE 5: LCR Using One 3' Phosphate Blocked Probe LCR was performed using probes AA123-1(20), AA123-2, AA123-3Q8) and AA123-4P(22) from Table I under conditions as in Example 3, above. The data is presented in Table E-5 below as a signal-to-background ratio. Table E-5: Signal-to-Background Ratio Endo IV Dilution Cycle No. 10 3 Targets ^ Targets 1:10000 28 1 3.3 31 2.4 5.8 34 2.3 3.8 1:5000 25 1 1.8 28 2.0 3.8 31 2.0 3.2 34 1.6 2.0 EXAMPLE 6: LCR Using Blunt-End 3' Phosphate Blocked Probes LCR was performed using the blunt-ended, 3'-phosphorylated probe set (see Table I) consisting of AA123-1P(20), AA123-2, AA123-3(20) and AA123-4P(20) in a 20 μL reaction volume containing LCR Buffer supplemented with 50 μg mL BSA, 0.5 mM CoCl 2 and 300 ng of human placental DNA. Each Probe was present at 83 nM (approximately 5% of probe 2 was 3'-end labeled with [α- 32 P]-cordycepin triphosphate to enable detection) and the enzymes Tth DNA ligase and Tth endonuclease IV were present at 0.15 μg/mL and 4.1 μg/mL respectively. Duplicate reactions for zero, IO 2 , and 10 3 molecules of target DNA were performed. The samples were overlaid with 15 μL of mineral oil and the temperature cycle consisted of a 95°C incubation for 30 seconds followed by a 55°C incubation for 110 seconds. At specified cycles (see Table E-6), 1.4 μL aliquots were removed, mixed with 2.0 μL of stop buffer, heated to 90° C for 2 min and applied to a denaturing polyacrylamide gel. Table E-6 is the average percent ligated and the ratio of (+)/(-) target for the duplicate reactions. It is observed that 10 3 targets are distinguishable from zero targets. Table E-6: Blunt-End 3'-PO 4 Probes Percent Ligated (+Target -Target) Cyclg NQ, 0 Targets 1Ω 2 arg ts Hft Targets 31 0.91 1.90 (2.1) 2.07 (2.3) 34 1.50 2.04 (1.4) 4.24 (2.8) 37 2.34 4.40 (1.9) 9.84 (4.2) EXAMPLE 7: LCR Using Non-blunt 3' Phosphate Blocked Probes LCR was performed using the non-blunt probe set (see Table I) consisting of AA123-1P(20), AA123-2, AA123-3(18) and AA123-4P(22) in a 20 μL reaction volume containing LCR Buffer supplemented with 10 μg mL BSA, 0.5 mM CoCl 2 and 300 ng of human placental DNA. Each Probe was present at 83 nM (approximately 5% of probe 2 was 3'-end labeled with [α- 32 P]-cordycepin triphosphate to enable detection) and the enzymes Tth DNA ligase and Tth endonuclease IV were present at 0.15 μg/mL and 4.1 μg/mL respectively. Duplicate reactions for zero, IO 2 , and 10 3 molecules of target DNA were performed. The samples were overlaid with 15 μL of mineral oil and the temperature cycle consisted of a 95°C incubation for 30 seconds followed by a 55"C incubation for 110 seconds. At specified cycles (see Table E-5), 1.4 μL aliquots were removed, mixed with 2.0 μL of stop buffer, heated to 90°C for 2 min and applied to a denaturing polyacrylamide gel. Table E-7 is the average percent ligated and the ratio of (+)/(-) target for the duplicate reactions. It is observed that 10 3 targets are distinguishable from zero targets. Table E-7: Overhang 3'-PO 4 Probes Percent Ligated (+Target -Target) Cvcle No. 0 Targets Hg Targets 10^ Targets 28 0.75 0.88 (1.2) 2.18 (2.9) 31 2.15 2.48 (1.2) 5.66 (2.6) 34 4.67 6.02 (1.3) 11.48 (2.5) 37 8.21 9.45 (1.2) 16.69 (2.1) EXAMPLE 8: LCR Using Probes Modified with an Abasic Site and an Extension of One Base. LCR was performed using the non-blunt probe set (see Table I) consisting of AA123-1E1, AA123-2, AA123-3(18) and AA123-4E1 in a 20 μL reaction volume containing LCR Buffer supplemented with 50 μg/mL BSA, 0.5 mM CoCl and 300 ng of human placental DNA. Each probe was present at 83 nM (approximately 5% of probe 2 was 3'-end labeled with [α- 32 P]-cordycepin triphosphate to enable detection) and the enzymes Tth DNA ligase and Tth endonuclease IV were present at 0.15 μg/mL and 41 μg mL respectively. Duplicate reactions for zero, 10 3 , and IO 4 molecules of target DNA were performed. The samples were overlaid with 15 μL of mineral oil and the temperature cycle consisted of a 95°C incubation for 30 seconds followed by a 55°C incubation for 240 seconds. At specified cycles (see Table E-8), 1.4 μL aliquots were removed, mixed with 2.0 μL of stop buffer, heated to 90°C for 2 min and applied to a denaturing polyacrylamide gel. Table E-8 is the average percent ligated and the ratio of (+)/(-) target for the duplicate reactions. It is observed that 10 3 targets are distinguishable from zero targets. Table E-8: Abasic Probes with a One Base Extension Percent Ligated Cycle o Targets πft Targets lift Targets 35 0 0.98 6.99 40 0 4.07 13.96 45 0 6.27 19.23 50 0 8.74 23.50 EXAMPLE 9: LCR Using Endonuclease IV Activity Isolated From Sulfolobus solfataricus PART A: Enzyme Isolation: Sulfolobus solfataricus (ATCC 35091) was grown as suggested by the ATCC. 20 grams of cells frozen in media were thawed and mixed with 10 mL of 50 mM Tris pH 7.4, 5% (w:v) glycerol, 0.5 mM dithiothreitol to yield a volume of 70 mL. 3.0 mL of 1.0 M Tris pH 7.4 was added and the cells were crushed by two passages through a French press at 14,000 psi. The mixture was centrifuged for 30 min. at 40,000 g. Approximately 66 mL of supernatant was collected, diluted with 0.5 volumes of glycerol and stored at -20°C. 20 mL of the lysate was diluted with 50 mL of 20 mM potassium phosphate pH 7.0, 1.0 mM dithiothreitol, 5% glycerol (v:v) ("Buffer A") and loaded onto a 1.6x9.5 cm column of Blue Sepharose (Pharmacia) equilibrated with Buffer A. The column was washed with 15 mL Buffer A and developed with a linear gradient from 0.1M to 0.74M NaCl in Buffer A. 3.2 mL fractions were collected and 1.25 mL of glycerol were added to each of 45 fractions collected. The fractions were tested for endonuclease IV activity employing the oligonucleotide E4SUB 1 (see Table III) labelled with [α- 32 P] cordycepin by terminal deoxynucleotidyl transferase and resolving the cut and uncut oligos on a denaturing polyacrylamide gel. Fractions 27-44 were pooled, made 0.15 mM in CoCl 2 , and heated to 80°C for 2.5 min. yielding a final volume of 59 mL. To concentrate the sample it was necessary to remove debris by centrifuging at 28,000 g for 15 min., passing the supernatant through a 0.2 μM filter and finally concentrating 3-fold in an Amicon Centriprep 10 filter. 3.0 mL of the concentrated protein solution was diluted with 30.0 mL of 25 mM Tris pH 7.4, 1.0 mM MgSO 4 , 50 μM CoCl 2 and 5% (v:v)glycerol ("Buffer B") and loaded onto a 4.0 mL heparin-agarose column equilibrated with Buffer B. The column was developed using a gradient of 0.25 M to 0.5 M KCl in Buffer B collecting 1.0 mL fractions. Fractions were assayed for abasic nuclease activity and fractions 9-11 were pooled. The 3.0 mL were transferred to 25 mM EPPS pH 7.7, 0.1 M KCl, 1 mM MgCl 2 , 50 μM CoCl 2 and 5% (v:v) glycerol ("Buffer C") by passing the sample over a BioRad 10DG column equilibrated with Buffer C. 4.0 mL was collected and concentrated to ca. 100 μL in an Amicon Centriprep 10. This sample was heated to 90°C for 5 min and was found to be free of both single- and double-stranded nuclease activity using oligodeoxyribonucleotides AA250-1 and AA250-3 as substrates. PART B : LCR Reactions: LCR was performed comparing the blunt-ended probe set consisting of AA250-1, AA250-2, AA250-3(24) and AA250-4(19) with the non-blunt probe set with one abasic probe consisting of AA250- 1E5, AA250-2, AA250- 3(22) and AA250-4(21) in a 20 μL reaction volume containing LCR Buffer supplemented with 10 μg/mL BSA, 2.0 mM CoCl 2 and 300 ng of human placental DNA. Each probe was present at 83 nM (approximately 5% of probe 2 was 3'-end labeled with [α- 32 P]- cordycepin triphosphate to enable detection) and the Tth DNA ligase was present at 0.15 μg/mL. The purified Sulfolobus solfataricus endonuclease IV activity was used at a final dilution of 1 : 10. Duplicate reactions for zero and IO 6 molecules of target DNA were performed for both blunt and non-blunt probe sets. The samples were overlaid with 15 μL of mineral oil and the temperature cycle consisted of a 95°C incubation for 30 seconds followed by a 55"C incubation for 60 seconds. At specified cycles (see Table E-9), 1.4 μL aliquots were removed, mixed with 2.0 μL of stop buffer, heated to 90°C for 2 min and applied to a denaturing polyacrylamide gel. Table E-9 gives the average percent ligated and the ratio of IO 6 targets/zero targets for the duplicate reactions in each case. It is observed that the ratio of IO 6 targets/zero targets is better with the abasic extension modified non-blunt probe set. This improvement is due to a decrease in the signal arising in the reactions containing zero target molecules whereas the percent ligated in the reactions containing IO 6 target molecules is the same for both probe sets. Table E-9: LCR with Endo IV from Sulfolobus solfataricus Cvcle No. 4th Probe 0 Targets 102 Target! 5 Ratio (+/-. Target 30 AA123-1 3.9 24.1 6.2 30 AA123-1E5 0.5 26.1 54.0 35 AA123-1 6.8 38.0 5.6 35 AA123-1E5 3.1 35.8 11.7 40 AA123-1 18.7 51.1 2.7 40 AA1E5 15.3 52.8 3.5 Examples 10-13 relate to the effect of divalent cations on LCR and Endonuclease IV modified LCR. EXAMPLE 10: Effect of MnCl 2 on LCR and the Improvement with Endonuclease IV LCR was performed using the non-blunt probe set (see Table I) consisting of AA123-1P(20), AA123-2, AA123-3(18) and AA123-4P(22) in a 20 μL reaction volume containing LCR Buffer supplemented with 10 μg mL BSA, 0.1 mM MnCl 2 and 300 ng of human placental DNA. In this experiment probe AA 123- 1P(20) was haptenated with biotin at its 5' terminus and AA123-2(20) was haptenated at its 3' terminus with digoxigenin for a purpose not related to this experiment. Each probe was present at 83 nM (approximately 5% of probe 3 was 3'-end labeled with [α- 32 P]-cordycepin triphosphate to enable detection) and the enzymes Tth DNA ligase and Tth endonuclease IV were present at 0.15 μg/mL and 4.1 μg/mL respectively. Duplicate reactions for zero and 10 3 molecules of target DNA were performed. The samples were overlaid with 15 μL of mineral oil and the temperature cycle consisted of a 95°C incubation for 30 seconds followed by a 55 β C incubation for 110 seconds. At specified cycles (see Table E-10), 1.4 μL aliquots were removed, mixed with 2.0 μL of stop buffer, heated to 90° C for 2 min and applied to a denaturing polyacrylamide gel. Table E-10 gives the average percent ligated and the ratio of (+)/(-) target for the duplicate reactions. It is observed that 10 3 targets are not distinguishable from zero targets. Table E-10: Overhang 3'-PO 4 Probes with MnCl 2 Percent Ligated Cvcle No. 0 Targets !(£ Targets Ratio (+/- Target 25 3.29 2.47 0.8 35 23.65 23.30 1.0 EXAMPLE 11: Effect of CoCl 2 Concentrations on LCR with Unmodified, Non-blunt Probes LCR was performed using the unmodified, non-blunt probe set (see Table I) consisting of AA123-1(20), AA123-2, AA123-3(18) and AA123-4(22) in a 20 μL reaction volume containing LCR Buffer supplemented with various amounts of CoCl 2 as indicated in Table E-l 1, 10 μg/mL BSA and 300 ng of human placental DNA. In this experiment probe AA123-1P(20) was haptenated with biotin at its 5' terminus and AA 123-2(20) was haptenated at its 3' terminus with digoxigenin. Each probe was present at 83 nM (approximately 5% of probe 3 was 3 '-end labeled with [α- 32 P]-cordycepin triphosphate to enable detection) and the final concentration of Tth DNA hgase equalled 0.15 μg mL. Duplicate reactions containing either zero or IO 6 molecules of target DNA were performed. The samples were overlaid with 10 μL of mineral oil and the temperature cycle consisted of a 95°C incubation for 30 s followed by a 55 °C incubation for 110 s. At 20 and 30 cycles 1.4 μL aliquots were removed, mixed with 2.0 μL of stop buffer, heated to 90°C for 2 min and applied to a denaturing polyacrylamide gel. Table E- 11 is the average percent ligated and the ratio of (+)/(-) target for the duplicate reactions. It is observed that CoCl 2 has an inhibitory effect on the extent of amplification in the reactions both with and without target DNA. This implies that CoCl 2 has an inhibitory effect on Tth DNA hgase. Table E-l 1: Effects of CoCl 2 Concentration on LCR Percent Ligated CoCl 2 Cvcle No. (mM) 0 Targets 106 Targets Ratio (+/-) Target 20 0.0 0.19 4.60 24.2 0.5 0.32 3.13 9.8 1.0 0.37 0.90 2.4 2.0 0.0 0.0 N.A. 30 0.0 12.28 22.42 1.8 0.5 2.68 12.96 4.8 1.0 0.75 5.88 7.8 2.0 0.0 0.0 N.A. EXAMPLE 12: LCR with Modified and Unmodified Probes in the Absence of MgCl 2 and CoC LCR was performed in a buffer containing 47 mM EPPS pH 7.8, 80 mM KCl, 10 mM HjCl, 5 mM MnCl , 10 μg/mL BSA and 15 μg/mL human placental DNA and no MgCl or CoCl 2 . Four sets of probes were used in this experiment, representing 3'- phosphorylated ends, abasic sites with extensions of 3 or 5 bases, and no modifications, as follows (see Table I): unmodified, non-blunt set AA123-1(20), AA123-2, AA123-3(18), and AA123-4(22) 3'-phosphorylated set AA123-1P(20), AA123-2, AA123-3(18), and AA123-4P(22) abasic extension set AA123-1E3, AA123-2, AA123-3(18), and AA123-4E3 abasic extension set AA 123- 1E5, AA 123-2, AA 123-3( 18), and AA 123-4E5 Duplicate reactions containing either zero or 10 4 molecules of target DNA were performed. The samples were overlaid with 15 μL of mineral oil and the temperature cycle consisted of a 95°C incubation for 30 s followed by a 55°C incubation for 110 s. After 60 cycles 3.0 μL aliquots were removed, mixed with 3.0 μL of stop buffer, heated to 90°C for 2 min and applied to a denaturing polyacrylamide gel. No amplification occurred with any of the probe sets indicating that MnCl 2 at 5 mM cannot substitute for 10 mM MgCl supplemented with low concentrations of either CoCl 2 or MnCl 2 . Analogous LCR assays incorporating the same buffer as above but with 10 mM MnCl 2 and an unmodified probe set consisting of AA 123- 1(20), AA123-2, AA123-3(18) and AA123-4(22) also showed no amplification out to 44 cycles. EXAMPLE 13: LCR and Endo IV LCR with varying Cobalt Concentrations. Endo IV-LCR assays were performed in a reaction mix consisting of 50mM EPPS pH7.8, 5mM MgCl 2 , 20μg ml BSA, 1 x IO 12 of the oligos given in Table IV below, 215 units Thermus thermophilus DNA ligase, a 1.5 x IO -4 dilution of T. thermophilus endonuclease IV, and various concentrations of CoQ 2 ranging from 500μM to 2 mM in a final reaction volume of 20 microliters. The oligos are specific for positions 6693-6739 of the Chlamydia trachomatis cryptic plasmid as given by Hatt, C. et al. Nuc. Acids Res. 16:4053-4067 (1988). Oligos #1 and #4 include 3' phosphate blocking groups as shown. Table IV. DNA PROBE SETS TO Position 6693-6739 of Chlamydia trachomatis cryptic plasmid Designation SEQUENCE SEO ID No. #1 5 ' Fl-GATACTTCGCATCATGTGTTCCp 3 ' 35 #3 5 ' pGGAGTTTCTTTGTCCTCCTATAACG-Bio 3 ' 36 #2 3 ' Fl-CTATGAAGCGTAGTACACAAGGp 5 ' 37 #4 3 ' pCCTCAAAGAAACAGGAGGATATTGC-Bio 5 ' 38 where: p = 3' - phosphate blocking group; and p = a normal 5' phosphate group; FI = a fluorescein moiety; and Bio = a biotin moiety Endo IV-LCR cycling conditions were 95 * C for 30 seconds then 55 * C for 110 seconds repeated thirty times in a Coy thermocycler. Negative reactions were set up with 300 nanograms of human placental DNA in dH 2 O. Positive reactions contained either 10^ or lθ6 molecules of a synthetic DNA oligonucleotide corresponding to map positions 6693-6739 of the C. trachomatis plasmid sequence in a background of 300 ng of placental DNA. Following amplification, reactions were diluted 1:1 with IMx diluent buffer, and the LCR amplification products were detected via a sandwich immunoassay performed using the Abbott IMx® automated immunoassay system. Results are shown in Table E- 13 below. It can be seen that at cobalt concentrations of 1.0 mM or less, IO 6 targets are distinguishable from no target; while at greater cobalt concentrations, target was not distinguishable. Table E- 13: IMx rate count Amount of C0CI2 concentration Target 0.5mM ImM 2mM 0 27.8 8.8 23.5 IO 4 47.3 8.9 19.8 lθ6 390.0 71.5 16.3 Examples 14-17 relate to the use of LCR reactions using endonuclease IV correctable modified probes which also contain ribonucleotide residues. The ribonucleotide residues permit selective destruction of the amplification products using RNase or alkali as a means for controlling possible contamination, as is taught in WO91/17270 for conventional LCR reactants. The probes used in these examples are shown in Table V below. Table V. Mixed DNA/RNA PROBE SETS TO Position 123 of ACTINOBACILLUS ACTINOMYCETEMCOMITANS De.signation SEQUENCE SEO ID No. AA123-1R(20) 5 'TTGTCGAGCACCTTGAATAA - 3 ' 25 AA123-1RP(20) 5 'TTGTCGAGCACCTTGAATAAp - 3 ' 26 AA123-2 5 " pTTAATGGCTTCGATTGGGCT- 3 ' 6 AA123-3(20) 3 'AACAGCTCGTGGAACTTATTp - 5 7 AA123-4R(20) 3 ' ΔATTACCGAAGCTAACCCGA- 5 ' 27 AA123-4RP(20) 3 " pΔATTACCGAAGCTAACCCGA- 5 ' 28 where: A is adenosine r/bønucleotide; p = 3' - phosphate blocking group; and p = a normal 5' phosphate group. EXAMPLE 14: Preparation of Mixed Ribo- and Deoxyribo-Oligonucleotides Having 3' Phosphate Blocking Groups The mixed ribo- and deoxyribo-oligonucleotides AA123-1R and AA123-4R (see Table V) were prepared by initiating synthesis on a solid phase support bearing an adenosine ribonucleotide. Synthesis was continued with the sequential addition of deoxyribonucleotide phosphoramidites using an automated DNA synthesizer. The resulting mixed oUgonucleotide ("ribo-modified" probe) was cleaved from the support and deprotected with 37% NH 4 OH at 55 °C for 12 hours and purified by reverse-phase HPLC on a C18 column. Radioactive phosphate moieties were added in two steps to make oligodeoxyribonucleotides having at the 3' terminal residue a 2'-OH and 3'- 32 PO 4 group. First, the ribo-modified probes AA123-1R or AA123-4R were incubated for 1.5 h at 37 °C with 20 pmol of [α- 32 P]-cordycepin 5 '-triphosphate (5000 Ci/mmol) and 10 units of deoxynucleotidyl terminal transferase in a total reaction volume of 15 μL buffered with 140 mM sodium cacodylate pH 7.2, 1 mM CoCl , 0.1 mM dithiothreitol. The unreacted cordycepin was separated from the oligonucleotide on a 1.0 mL Sephadex G-50 column equilibrated with 5 mM Tris pH 8.0, 0.1 mM EDTA. Two drop fractions (ca. 75 μL) were collected and the elution profile was monitored by counting 1.0 μL of each fraction in 4.0 mL of liquid scintillation cocktail. The fractions containing the oligonucleotide were pooled. The resulting oligonucleotide contains a single 32 P-labeled 3'- phosphodiester bond adjacent to the unique 2'-OH group at the 3' terminus. Second, the phosphodiester bond between the oligonucleotide and the cordycepin was cleaved with T2 RNase, which cuts 3' to the adenosine residue, liberating the cordycepin deoxyribonucleoside and the desired ribo-modified probes. Approximately 0.2 pmol of 32 P labeled AA123-1R or AA123-4R were incubated with 2.5 units of T2 RNase for 1 h at 37 °C in a total reaction volume of 4.0 μL buffered with 50 mM potassium acetate, pH 5.2. The reaction was terminated by adding 5.0 μL of stop buffer and 1.0 μL of 5' LCR buffer and 5.0 μL was loaded on a 20x40x0.04 cm denaturing 12.5% polyacrylamide gel and electrophoresed for 1.75 h at 30W. As can be observed in Figure 5 Oanes 3 and 11), the products AA123-1RP(20) and AA123-4RP(20), resulting from RNase digestion of the [α- 32 P]-cordycepin labeled AA123-1R and AA123-4R migrate faster than the undigested oligonucleotides (lanes 2 and 10). Although the products of digestion with T2 RNase are expected to be decreased in length by only one base, they are observed to migrate with an Rf value similar to that of the faint failure sequence that is two bases shorter than the undigested material. This is expected since the correlation between length and Rf values of DNA fragments is based on all the DNA fragments having the same mass to charge ratio but, the additional negative charge associated with the 3'-phosphorylated oligonucleotide will increase the charge to mass ratio, resulting in an increased Rf value. EXAMPLE 15: Activity of Endonuclease IV on Ribo-Modified Probes The 3'-phosphorylated oligonucleotides AA123-1RP(20) and AA123-4RP(20) were further analyzed to determine if the 3'-PO 4 group attached to a ribonucleoside could be removed by Tth endonuclease IV, and if the enzymatic removal required that AA123- 1RP(20) and AA123-4RP(20) be hybridized to the complementary synthetic target oligonucleotides AA123TAR(l/2) or AA123TAR(3/4), respectively, (see Table m) so that the 3'-PO 4 is located in a double-stranded region. This was achieved by digesting aliquots of the [α- 2 P] -cordycepin labeled AA123-1R and AA123-4R with T2 RNase, as in example 14, followed by a second digestion with Tth endonuclease IV in the presence or absence of AA123TAR(l/2) or AA123TAR(3/4) in LCR buffer. As required, 1.7 pmol of AA123TAR(l/2) or AA123TAR(3/4) and Tth endonuclease IV, to a final concentration equal 4.1 μg mL, were added and the volume was adjusted to 10 μL with water. The samples were incubated for 1 h at 55 °C, 5.0 μL aliquots were removed into 5.0 μL of stop buffer, and then analyzed by denaturing polyacrylamide gel electrophoresis as detailed above. As can be seen in Figure 5 (lanes 4 and 12), the removal of the 3'- 32 PO 4 from the oligonucleotide occurs only when T2 RNase, Tth endonuclease IV and the strand complementary to the labeled oligonucleotide are all present. The absence of any one of these three materials prevents the removal of the 32 P label from AA123-1RP(20) and AA123-4RP(20). It has also been observed (data not shown) that the cleavage of AA123-1RP(20) by T2 RNase in the presence of target AA123TAR(l/2) but the absence of Tth endonuclease IV results in the same size product as observed with T2 RNase in the absence of the complementary target. These results demonstrate the reaction product from T2 RNase treatment of the [α- 32 P]-cordycepin labeled oligonucleotide, which should be a 3'-PO 4 group, is indeed a suitable substrate for endonuclease IV and obeys the same double-stranded substrate specificity described for endonuclease IV with DNA substrates. EXAMPLE 16: Demonstration of Ligation of Endonuclease IV Product Part A. 3' phosphorylated probes were synthesized by initiating automated synthesis with 2-[[2-[(4,4'-dimethoxytrityl)oxy]ethyl] sulfonyl]ethyl 2-cyano-ethyl N,N- diisopropylphosphoramidite (Horn, T and Urdea, M Tet. Lett. 27 4705+ (1986)) as reported (Ashely, GW and Kushland, DM Biochemistry 30: 2927-2933 (1991)), followed by the sequential addition of ribonucleotide and deoxyribonucleotide cyanoethyl phosphoramidites using an automated DNA synthesizer. The oligonucleotide was cleaved from the support and deprotected with 37% NHjOH at 55 °C for 12 h and purified by reverse-phase HPLC on a C18 column. Part B. It was then demonstrated that when the 3 '-phosphate is removed by Tth endonuclease IV (as in example 15) the resulting ribo-modified oligonucleotide is a suitable substrate for hgation by Tth DNA ligase. Duplicate reactions containing 83 nM AA123-1RP(20) from example 14, 16 nM AA123-2 (approximately 25% 3'-labeled with [α- 32 P]-cordycepin triphosphate), and 66 nM AA123TAR(l/2) in LCR Buffer supplemented with 2.0 mM CoCl 2 and 10 μg/mL BSA were incubated in the presence or absence of 0.15 μg/mL Tth DNA hgase and/or 4.1 μg mL Tth endonuclease IV at 55 °C for 1 h. Analogous assays containing 300 ng of human placental DNA and no AA123TAR(l/2) were also performed. As can be observed in Figure 6 (lanes 3 and 4), almost all of the AA123-2 is converted into a ligated product only when Tth endonuclease IV, Tth DNA ligase and AA123TAR(l/2) are all present. It was also observed (Fig. 6, lanes 1 and 2) that a small amount of ligated product was formed in the presence of AA123TAR(l/2) and hgase but the absence of Tth endonuclease IV. The formation of a ligated product in the absence of Tth endonuclease IV implies that the 3' position of AA123-1RP is not completely blocked with a 3'-PO 4 group. A 3'-OH group may arise from incomplete 3'-phosphorylation during synthesis or the removal of and\or exchange of the 3'-PO group with the 2-OH group during the treatment with strong alkali that follows synthesis. If the exchange reaction is responsible for generating an unblocked 3'-OH group then it must be possible for Tth DNA ligase to use the 2'-PO 4 , 3'-OH ribonucleoside as a substrate. EXAMPLE 17: LCR Using Modified Probes Containing a 3' R bonucleotide Bearing a 3' Phosphate LCR was performed using the blunt-end probe set (see Table V) consisting of AA123-1RP(20), AA123-2, AA123-3(20) and AA123-4RP(20) in a 20 μL reaction volume containing LCR Buffer supplemented with 10 μg/mL BSA, 0.5 mM CoCl 2 and 300 ng of human placental DNA. Each probe was present at 83 nM (approximately 5% of probe 2 was 5'-end labeled with [γ- 32 P]-adenosine triphosphate to enable detection) and the enzymes Tth DNA hgase and Tth endonuclease IV were present at 0.15 μg/mL and 4.1 μg mL, respectively. Duplicate reactions for zero, 10 3 , and IO 4 molecules of target DNA were performed. The samples were overlaid with 15 μL of mineral oil and the temperature cycle consisted of a 95 °C incubation for 30 seconds followed by a 55 °C incubation for 110 seconds. At specified cycles (see Table E-17), 1.7 μL aliquots were removed, mixed with 2.5 μL of stop buffer, heated to 90 °C for 2 min and applied to a denaturing polyacrylamide gel. Table E-17 gives the average percent ligated and, in parentheses, the ratio of (+)/(-) target for the duplicate reactions. It is observed that 10 3 targets are distinguishable from zero targets. Table E-17: LCR with 3 * -Ribo; 3'-PO 4 Probes Percent Ligated (+Target/-Target) Cycle No. 0 targets l _ύ targets lθ targets 28 0.45 1.65 (3.7) 5.87 (13.0) 31 1.83 5.50 (3.0) 12.08 (6.6) 34 6.70 12.87 (1.9) 21.60 (3.2) 37 13.35 23.39 (1.8) 28.49 (2.1) For Examples 18 and 19 plasmid pUC19 is used as target, hence the designation "pUC" in Table VI, below. The next numeral (after the dash) designates the position of the probe in a set of four: Probes numbered -1 and -2 have the same 5 '-3' orientation while probes numbered -3 and -4 have the opposite sense. Probes numbered -1 and -3 hybridize, as do probes numbered -2 and -4. "P" and "p" indicate a phosphate group. This is normal and required on a 5' terminus, but serves as a ligation blocking modification on a 3' end. "E" and "x" designate an abasic site (described further below). A number "1" following the "E" designates the length of complementary bases (overhang) beyond the abasic site. Table VI; pUC19 Modified Probes Designation SEQUENCE & ORIENTATION SEO ID No. pUC-lP 5 ' -AATTCGAGCTCGGTACCCp -3' 29 pUC-lEl 5 ' -AATTCGAGCTCGGTACCCxG -3 ' 30 pUC-2 5' - pGGGGATCCTCTAGAGTCGACCTGCA --33 '' 31 pUC-3 3 ' - GCTCGAGCCATGGGp -5 ' 32 pUC-4P 3' - pCCCCTAGGAGATCTCAGCTG -5' 33 PUC-4E1 3 ' - GxCCCCTAGGAGATCTCAGCTG -5 ' 34 EXAMPLE 18: LCR Using a pUC19 Target with 3' Phosphate Blocked Probes A probe set is designed to detect the pUC19 target sequence by LCR, with reduced background levels. The probe set (see Table VI) features two normal probes (pUC-2 and pUC-3) two probes (pUC-lP and pUC-4P) containing terminal 3' phosphate blocking groups. LCR reactions are performed (substantially as described herein) using various amounts of target (pUC19). After the hybridization step of each cycle, endonuclease IV purified from E. coli is added to the reaction. This can be done under standard LCR conditions, since E coli endonuclease IV is somewhat thermostable. Alternatively, endonuclease IV from a thermostable species such as Thermus thermophilus could be used. As a control, the LCR is run using the same number of target molecules without the addition of endonuclease IV. In these controls a probe set similar to the one shown above is used, only the 3* terminal nucleotides (containing the 3' phosphates) are not included on probes -1 and -4. In both the experimental and control reactions, the rate of appearance of ligated product is correlated with the initial number of target molecules added. What distinguishes the two protocols is that in the second case, a "blank" tube containing no target molecules will give rise to signal at about the same rate as a tube containing 1000 target molecules, whereas in the case where modified probes and endonuclease IV are used, a "blank" tube containing no target molecules will give rise to signal significantly more slowly than does a tube containing 1000 target molecules. This suppression of -36- background provides an advantage in increasing the usable range of sensitivity of the assay. Those skilled in the art will immediately appreciate the desirability of employing a highly thermostable endonuclease IV, for the same reasons that highly thermostable ligases and polymerases are useful and desirable in LCR and PCR, respectively. Those skilled in the art will also appreciate that other enzymes, either known or not yet known, which can remove modifications at the 5' or 3' ends of a DNA strand in a template dependent manner leaving the previously blocked 5' phosphate or 3 * hydroxyl intact, can be employed in a manner completely analogous to endonuclease IV as described above. EXAMPLE 19: LCR Using a ρUC19 Target with Abasic Probes A probe set is designed to detect the pUC19 target sequence by LCR, with reduced background levels. The probe set (see Table VI) features two normal probes (pUC-2 and pUC-3) and two modified probes (pUC-lEl and pUC-4El) containing a 3' end abasic site followed by one additional normal residue complementary to the target LCR reactions are performed as in example 18. Results and interpretation will be similar to those of example 18. SEQUENCE LISTING (1) GENERAL INFORMATION: (i) APPLICANT: Keith C. Backman John J. Carrino George H. Shimer Robert R. Yocum (ii) TITLE OF INVENTION: LIGASE CHAIN REACTION WITH ENDONUCLEASE IV CORRECTION AND CONTAMINATION CONTROL (iii) NUMBER OF SEQUENCES: 38 (iv) CORRESPONDENCE ADDRESS: (A) ADDRESSEE: Abbott Laboratories (B) STREET: One Abbott Park Road (C) CITY: Abbott Park (D) STATE: Illinois (E) COUNTRY: USA (F) ZIP: 60064-3500 (v) COMPUTER READABLE FORM: (A) MEDIUM TYPE: Floppy diskette (B) COMPUTER: IBM PC compatible (C) OPERATING SYSTEM: PC-DOS/MS-DOS (D) SOFTWARE: WordPerfect (vi) CURRENT APPLICATION DATA: (A) APPLICATION NUMBER: (B) FILING DATE: (C) CLASSIFICATION: (viii) ATTORNEY/AGENT INFORMATION: (A) NAME: Thomas D. Brainard (B) REGISTRATION NUMBER: 32,459 (C) REFERENCE/DOCKET NUMBER: 4773.PC.04 (ix) TELECOMMUNICATION INFORMATION: (A) TELEPHONE: 708-937-4884 (B) TELEFAX: 708-938-2623 (2) INFORMATION FOR SEQ ID NO: 1: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 20 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 1: TTGTCGAGCA CCTTGAATAA 20 (3) INFORMATION FOR SEQ ID NO: 2: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 20 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (ix) FEATURE: (A) NAME/KEY: 3' end phosphorylated (B) LOCATION: 20 (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 2: TTGTCGAGCA CCTTGAATAA 20 (4) INFORMATION FOR SEQ ID NO: 3: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 22 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (ix) FEATURE: (A) NAME/KEY: N represents an abasic site (B) LOCATION: 21 (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 3: TTGTCGAGCA CCTTGAATAA NT 22 (5) INFORMATION FOR SEQ ID NO: 4: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 24 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (ix) FEATURE: (A) NAME/KEY: N represents an abasic site (B) LOCATION: 21 (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 4: TTGTCGAGCA CCTTGAATAA NTAA 24 (6) INFORMATION FOR SEQ ID NO: 5: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 26 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (ix) FEATURE: (A) NAME/KEY: N represents an abasic site (B) LOCATION: 21 (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 5: TTGTCGAGCA CCTTGAATAA NTAATG 26 (7) INFORMATION FOR SEQ ID NO: 6: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 20 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 6: TTAATGGCTT CGATTGGGCT 20 (8) INFORMATION FOR SEQ ID NO: 7: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 20 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 7: TTATTCAAGG TGCTCGACAA 20 (9) INFORMATION FOR SEQ ID NO: 8: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 18 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 8: ATTCAAGGTG CTCGACAA 18 (10) INFORMATION FOR SEQ ID NO: 9: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 22 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (ix) FEATURE: (A) NAME/KEY: 3' end phosphorylated (B) LOCATION: 22 (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 9: AGCCCAATCG AAGCCATTAA TT 22 (11) INFORMATION FOR SEQ ID NO: 10: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 20 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (ix) FEATURE: (A) NAME/KEY: 3' end phosphorylated (B) LOCATION: 20 (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 10: AGCCCAATCG AAGCCATTAA 20 (12) INFORMATION FOR SEQ ID NO: 11: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 20 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 11: AGCCCAATCG AAGCCATTAA 20 (13) INFORMATION FOR SEQ ID NO: 12: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 24 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (ix) FEATURE: (A) NAME/KEY: N represents an abasic site (B) LOCATION: 23 (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 12: AGCCCAATCG AAGCCATTAA TTNT 24 (14) INFORMATION FOR SEQ ID NO: 13: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 26 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (ix) FEATURE: (A) NAME/KEY: N represents an abasic site (B) LOCATION: 23 (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 13: AGCCCAATCG AAGCCATTAA TTNTTC 26 (15) INFORMATION FOR SEQ ID NO: 14: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 28 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (ix) FEATURE: (A) NAME/KEY: N represents an abasic site (B) LOCATION: 23 (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 14: AGCCCAATCG AAGCCATTAA TTNTTCAA 28 (16) INFORMATION FOR SEQ ID NO: 15: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 25 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 15: CCGATTGCAA TGTAATATCG ACGTC 25 (17) INFORMATION FOR SEQ ID NO: 16: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 31 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (ix) FEATURE: (A) NAME/KEY: N represents an abasic site (B) LOCATION: 26 (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 16: CCGATTGCAA TGTAATATCG ACGTCNTCGG C 31 (18) INFORMATION FOR SEQ ID NO: 17: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 21 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 17: GTCGGGCAAA TAATTCGCCA C 21 (19) INFORMATION FOR SEQ ID NO: 18: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 24 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 18: GACGTCGATA TTACATTGCA ATCG 24 (20) INFORMATION FOR SEQ ID NO: 19: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 22 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 19: CGTCGATATT ACATTGCAAT CG 22 (21) INFORMATION FOR SEQ ID NO: 20: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 19 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 20: GGCGAATTAT TTGCCCGAC 19 (22) INFORMATION FOR SEQ ID NO: 21: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 21 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 21: GGCGAATTAT TTGCCCGACG A 21 (23) INFORMATION FOR SEQ ID NO: 22: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 40 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 22: TTGTCGAGCA CCTTGAATAA TTAATGGCTT CGATTGGGCT 40 (24) INFORMATION FOR SEQ ID NO: 23: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 40 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 23: AGCCCAATCG AAGCCATTAA TTATTCAAGG TGCTCGACAA 40 (25) INFORMATION FOR SEQ ID NO: 24: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 45 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (ix) FEATURE: (A) NAME/KEY: N represents an abasic site (B) LOCATION: 36 (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 24: AAAAAAAGCC GGATCCGTAC ACAACGGATC CGGCTNTTTT TGGGG 45 (26) INFORMATION FOR SEQ ID NO: 25: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 20 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (ix) FEATURE: (A) NAME/KEY: adenosine ribonucleotide (B) LOCATION: 20 (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 25: TTGTCGAGCA CCTTGAATAA 20 (27) INFORMATION FOR SEQ ID NO: 26: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 20 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic) (ix) FEATURE: (A) NAME/KEY: adenosine ribonucleotide (B) LOCATION: 20 (ix) FEATURE: (A) NAME/KEY: 3' end phosphorylated (B) LOCATION: 20 (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 26: TTGTCGAGCA CCTTGAATAA 20 (28) INFORMATION FOR SEQ ID NO: 27: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 20 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (ix) FEATURE: (A) NAME/KEY: adenosine ribonucleotide (B) LOCATION: 20 (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 27: AGCCCAATCG AAGCCATTAA 20 (29) INFORMATION FOR SEQ ID NO: 28: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 20 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (ix) FEATURE: (A) NAME/KEY: adenosine ribonucleotide (B) LOCATION: 20 (ix) FEATURE: (A) NAME/KEY: 3' end phosphorylated (B) LOCATION: 20 (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 28: AGCCCAATCG AAGCCATTAA 20 (30) INFORMATION FOR SEQ ID NO: 29: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 18 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (ix) FEATURE: (A) NAME/KEY: 3' end phosphorylated (B) LOCATION: 18 (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 29: AATTCGAGCT CGGTACCC 18 (31) INFORMATION FOR SEQ ID NO: 30: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 20 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (ix) FEATURE: (A) NAME/KEY: N represents an abasic site (B) LOCATION: 19 (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 30: AATTCGAGCT CGGTACCCNG 20 (32) INFORMATION FOR SEQ ID NO: 31: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 25 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 31: GGGGATCCTC TAGAGTCGAC CTGCA 25 (33) INFORMATION FOR SEQ ID NO: 32: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 14 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 32: GGGTACCGAG CTCG 14 (34) INFORMATION FOR -SEQ ID NO: 33: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 20 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (ix) FEATURE: (A) NAME/KEY: 3 1 end phosphorylated (B) LOCATION: 20 (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 33: GTCGACTCTA GAGGATCCCC 20 (35) INFORMATION FOR SEQ ID NO: 34: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 22 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (ix) FEATURE: (A) NAME/KEY: N represents an abasic site (B) LOCATION: 21 (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 34: GTCGACTCTA GAGGATCCCC NG 22 (36) INFORMATION FOR SEQ ID NO: 35: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 22 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (ix) FEATURE: (A) NAME/KEY: 3' end phosphorylated (B) LOCATION: 22 (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 35: GATACTTCGC ATCATGTGTT CC 22 (37) INFORMATION FOR SEQ ID NO: 36: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 25 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 36: GGAGTTTCTT TGTCCTCCTA TAACG 25 (38) INFORMATION FOR SEQ ID NO: 37: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 22 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 37: GGAACACATG ATGCGAAGTA TC 22 (39) INFORMATION FOR SEQ ID NO: 38: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 25 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: Other nucleic acid (synthetic DNA) (ix) FEATURE: (A) NAME/KEY: 3' end phosphorylated (B) LOCATION: 25 (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 38: CGTTATAGGA GGACAAAGAA ACTCC 25

Claims: What is claimed is: 1. In a ligase chain reaction method for amplifying a target nucleic acid sequence, said method including: (a) providing an excess of at least two sets of two probes, the 3 ' end of an upstream probe being ligated to the 5 ' end of a downstream probe in the presence of target to form a primary ligation product and the second set of probes hybridizing to the primary ligation product and being ligated to each other to form a secondary ligation product; (b) repeatedly denaturing the hybridized strands, reannealing additional probes and ligating them; and (c) detecting to what extent ligation products have formed, the improvement comprising: (a) providing in at least one of the upstream probes a 3 ' end modification such that the probe is incapable of ligation to its downstream partner, said 3' end modification being correctible substantially only when the modified probe is hybridized to the target sequence; (b) hybridizing the modified probe to the target, if present, to form a modified probe-template complex; (c) correcting the modification in a target dependent manner using endonuclease IV activity to create a 3' hydroxyl end, thus allowing the corrected probe to be ligated to its downstream partner, (d) ligating the corrected probe to its downstream partner to form an amplification product; and (e) dissociating the amplification product from the target and repeating the hybridization, correction and ligating steps to amplify the desired target sequence. 2. The method of Claim 1 wherein said 3' modification comprises a blocking moiety attached to said probe such that it blocks a chemical group required in the ligation step. 3. The method of Claim 2 wherein the blocking moiety is of the form: O II — P — o-z I OH wherein Z is selected from the group consisting of -H; -(CH2)n CHO, where n is from 1 to about 3; -deoxyribose; and-dideoxyribose. 4. The method of Claim 3 wherein said blocking moiety is phosphate. 5. The method of Claim 1, wherein at least said correcting step is performed in the presence of available divalent cation selected from cobalt and manganese at a concentration of at least about 0.05 mM 6. The method of Claim 5, wherein said divalent cation is at a concentration from about 0.1 to about 2.0 mM. 7. The method of Claim 5, wherein said divalent ion is cobalt at a concentration of about 0.5 mM to about 1.0 mM. 8. The method of Claim 5, wherein said divalent ion is manganese at a concentration of about 0.5 mM to about 1.0 mM. 9. The method of Claim 1, wherein one or more of the steps is performed in the presence of divalent magnesium ion at a concentration of about 0.5 mM to about 20 mM. 10. The method of Claim 1, wherein said two sets of two probes are oligodeoxyribonucleotide probes, except that at least one of said probes includes at least one ribonucleotide residue. 11. The method of Claim 10, wherein said at least one ribonucleotide residue occurs immediately 5' to the 3' modification. 12. The method of Claim 3, wherein said probe having the 3' end blocking moiety further includes at least one ribonucleotide residue, and the blocking moiety is attached to the 3' position of said ribonucleotide. 13. The method of Claim 10, further comprising, after the detection step, a step of cleaving ligation products using RNase or alkali. 14. The method of Claim 10, further comprising, prior to amplification, a step of cleaving ligation products using RNase. 15. The method of Claim 14, wherein the blocking moiety is of the form: O II — P — o-z I OH wherein Z is selected from the group consisting of -H; -(CH2)n CHO, where n is from 1 to about 3; -deoxyribose; and-dideoxyribose. 16. The method of Claim 2 wherein said blocking moiety is a nucleic acid overhang containing an abasic residue immediately 3' to the point of intended liagtion. 17. The method of Claim 16 wherein said correcting of the modification comprises cleavage of said modified probe on the 5' side of said abasic site, substantially only when said modified probe is hybridized to target or to ligation product. 18. The method of Claim 16, wherein said correcting step is performed in the presence of available divalent cobalt ion at a concentration of about 0.05 mM to about 2.0 mM. 19. The method of Claim 18, wherein said hybridizing, correcting and ligating steps are performed in the presence of available cobalt ion at a concentration of about 0.5 mM to about 1.0 mM. 20. The method of Claim 18, wherein said concentration of divalent Mn ion is at least about 0.05 mM. 21. The method of Claim 18, wherein said correcting step is performed in the presence of divalent magnesium ion at a concentration of about 0.5 mM to about 20 mM. 22. The method of Claim 16, wherein said two sets of two probes are oligodeoxyribonucleotide probes, except that at least one of said probes includes at least one ribonucleotide residue. 23. The method of Claim 22, further comprising, after the detection step, a step of cleaving ligation products using RNase or alkali. 24. The method of Claim 17, further comprising, prior to amplification, a step of cleaving ligation products using RNase. 25. The method of Claim 1, wherein said detecting is by means of a hapten marker attached to the outside ends of the primary upstream and secondary downstream probes and by a reporter or different hapten attached to the outside ends of the primary downstream and secondary upstream probes. 26. The method of Claim 25 wherein the reporter is a radioactive, fluorescent or chemiluminescent compound 27. The method of Claim 1 wherein the blocking moiety comprises a detectable label and said detecting is by means of monitoring the release of detectable label from the modified probe. 28. The method of Claim 2 wherein said upstream probe containing the 3' modification and its complementary probe are selected such that, when the two probes are hybridized together, the 5' terminal residue of the complementary probe lies opposite a nucleotide residue in the upstream probe which is at least one residue 5' to the site of the modification. 29. The method of Claim 16 wherein said upstream probe containing the 3' abasic site and its complementary probe are selected such that, when the two probes are hybridized together, the 5' terminal residue of the complementary probe lies opposite a nucleotide residue in the upstream probe which is at least one residue 5' to the abasic site. 30. A diagnostic kit comprising in combination: (a) two pairs of probes hybridizable with target such that they are capable of being ligated in LCR, wherein at least one of the probes is modified such that, when hybridized, a ligase is substantially incapable of acting on the modified probe as its substrate; (b) a first enzyme reagent having ligase activity for assembling an amplification product and (c) a second enzyme reagent having endonuclease IV activity capable of correcting the modified probe in a target dependent manner to allow the probe-template complex to be acted upon by the ligase reagent. 31. The kit of Claim 30, further including a buffer or means for preparing a buffer containing from 0.05 mM to about 2.0 mM of divalent cobalt ion. 32. The kit of Claim 30, further including an RNase reagent or an alkaline reagent. 33. The kit of Claim 30 wherein the first enzyme reagent, the second enzyme reagent, or both, are thermostable. 34. The kit of Claim 30 wherein the second enzyme reagent comprises Endonuclease IV. 35. A nucleic acid probe substantially free of natually occurring nucleic acid fragments, said probe having at least three deoxyribonucleotides covalentiy linked by phosphodiester linkages to define a 5' and a 3' end of the probe, the probe further comprising a ribonucleotide at the 3' end, the 3' position of the ribonucleotide having attached thereto a group of the formula: — P — o-z I OH wherein Z is selected from the group consisting of -H; -(CH2)n CHO, where n is from 1 to about 3; -deoxyribose; and-dideoxyribose. 36. The probe of Claim 35 wherein Z is hydrogen. 37. The probe of Claim 35 having about 12 to about 50 deoxyribonucleotides.

Inventors: BACKMAN KEITH C, CARRINO JOHN J, SHIMER GEORGE H, YOCUM ROBERT R, BACKMAN, KEITH, C., CARRINO, JOHN, J., SHIMER, GEORGE, H., YOCUM, ROBERT, R.
Applicants: ABBOTT LAB, ABBOTT LABORATORIES
Application year: 1994
Main classification: C12Q1
IPC codes: C07H21, C12N15, C12Q1